Senior police officers have strongly defended the radical extension of the role of private companies in policing, saying they should be involved in protecting the public and bringing offenders to justice.

The comments from the Association of Chief Police Officers follow the disclosure by the Guardian that the West Midlands and Surrey police authorities have invited private security companies to bid for a wide range of services, including criminal investigations, patrolling neighbourhoods and detaining suspects.

Greater Manchester chief constable Peter Fahy, Acpo's spokesman on workforce development, said that only "radical and fundamental" change would allow forces to cope with the "enormous challenge of the financial cuts" and maintain the protection of the public.

He said all forces were watching the £1.5bn West Midlands/Surrey tendering process very closely "to examine where the limits of the involvement of other bodies in policing should lie". For example, there were elements in a criminal investigation that did not need to be done by a police officer. Fahy said that the office of constable restricted the power of arrest to a warranted officer and was a fundamental safeguard for the public. But that did not mean that others could not help protect the public and bring offenders to justice.

The former Metropolitan police commissioner, Sir Ian Blair, writing in the Guardian said police forces need to modernise their budgets, "namely by reducing unit costs".

He says that "swaths of police tasks'' do not need to be carried out by fully trained officers. These include guarding prisoners, searching woodlands, preparing routine witness statements and providing intelligence analysis to murder inquiries.

"Many forces have employed their own non-police staff to undertake this sort of task but have been unable to do so in sufficient numbers because of the need to employ a fixed and ever increasing number of officers within a fixed budget.

"The tender offered by West Midlands and Surrey police signals a shift which would allow the private sector to provide staff who can carry out routine and repetitive tasks at cheaper rates and, perhaps most intriguingly, to provide temporary access to skilled staff – such as murder inquiry teams – which can be hired for incidents that are rare in most forces but for which all forces must permanently retain a group of very expensive staff. This would then allow the chief constable, satisfied that he or she has commissioned these kind of services at a cheaper rate, to spend more of the budget on those parts of the service that require, because of their complexity, their impact on public safety or their centrality to the police mission, to be carried out by fully warranted officers," he says.

Fahy tried to assuage public fears that outsourcing could lead to private companies carrying out investigations, patrolling or detaining suspects by pointing out that private security staff were already patrolling public spaces and managing major public events, licensed by local authorities: "Private staff monitor CCTV covering public space, private companies transport prisoners to and from court and store detectives detain shoplifters."

He said that chief officers needed to ensure that highly trained and professional police officers were spending time on activities which require their skills, expertise and values. "While there are a number of tasks in a criminal investigation, such as gathering CCTV evidence or checking phone records, which do not necessarily need to be done by a police officer, the investigation itself would be overseen by a police officer in much the same way as a doctor oversees treatment of a patient although other healthcare professionals carry out particular tasks," he said. The £1.5bn contract, which could be worth up to £3.5bn if other forces sign up, makes clear that nearly every police service is on the table except for those involving the powers of a warranted officer, including arrest. It will be up to each participating force which services are privatised.

A "bidder's conference" is due next week. It is expected that the contract would come into force early next year.

Bob Jones, chair of the finance committee of the West Midlands police authority and a former chair of the authority and the national organisation of police authorities, said that the trial was "very much" driven by central government, particularly Home Office minister Nick Herbert, and Cabinet Office minister and paymaster general Francis Maude.

Jones said he was one of five members of the police authority who voted against the trial, because he was unhappy that conditions they had requested were not met, including certainty about the success of other private police trials and more clarity before private companies bid for contracts about what roles they should play. "I haven't got a problem with the principle, providing it's properly accountable, well managed, it brings benefits and there are protections for staff – providing it's basically in the interests of the West Midlands," said Jones. "Given the dire financial position, it does need to be explored, but I don't think it's being explored in a proper business-like manner." The lack of clarity, and fears of privatisation creep as companies are given increasing powers over time, will be raised by Labour in the Commons at the next home secretary's questions, said Jack Dromey, MP for Birmingham Erdington. "Government are driving Britain's police service down the path of privatisation with virtually nothing ruled out by ministers at this stage."

Brian Paddick, the former Scotland Yard deputy assistant commissioner and Lib Dem candidate for London mayor, wrote on the Guardian's Comment is Free website: "The British tradition of policing by consent, rather than by force and weight of numbers, is being eroded, and plans to use private security firms to carry out core policing tasks, as reported this weekend, will accelerate that process."

Lynne Owens, the Surrey chief constable ruled out the use of private firms to patrol neighbourhoods: "Any suggestion that a private sector company will patrol the streets of Surrey is simply nonsense. It would be no more acceptable to the public than it would be to me," she said.

But although senior officers say that they are only talking about putting back and middle office functions out to contract to support frontline services many critics fear that it involves a radical shift in the dividing line between public and private in policing. The Police Federation warned that it was "an extremely dangerous road to take".

A Home Office spokesman said: "We are determined to do anything that will help the police to become more efficient and better able to fight crime. We have been very open in our support for the police in taking these decisions."